[Python-Dev] Explorations on new-style classes

[Kevin Jacobs]

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On Tue, 5 Mar 2002, David Abrahams wrote:
> Although I understand the appeal, using __slots__ to constrain the
> attributes might be misguided (depending on your goals):

How about something even more misguided?  I've implemented a metaclass that
works around the problem of layout conflicts.  e.g.:

  class A(object):
    __metaclass__ = LazySlotBuilder
    __slots__ = 'a'

  class B(object):
    __metaclass__ = LazySlotBuilder
    __slots__ = 'b'

  class C(B):
    __slots__ = 'c'

  class AB(A,B):
    __slots__ = ['ab']

  class ABC(A,B,C,AB):
    __slots__ = ['abc']

  abc=ABC()
  print type(abc).__slots__
  > ['a', 'c', 'b', 'ab', 'abc']


An implementation of LazySlotBuilder metaclass is attached.  It works by
creating 2 types instead of 1 if slots are present.  The first class is an
abstract version of the desired class with no slots declared in itself or
its base classes.  A concrete version is then built that inherits from the
abstract version, adding only the slot definitions and a reference to the
abstract class.

The current version has one significant limitation: it requires that all
base classes with slots have the LazySlotBuilder metaclass.  This can be
worked around, but makes the implementation needlessly complex.

I'm very much interested in what people think about these ideas about using
(or mis-using) slots, metaclasses, new-style classes, and other cutting-edge
Python features.  I am intentionally trying to push the limits and take
things beyond what some of these features were originally intended to do. 
It is my hope that such examples will educate, spur discussion and help
determine exactly which behaviors are intentional and supported, and which
are implementation artifacts that should not be taunted lightly.

I am also aware that a sizeable fraction of python-dev may not care much
about this topic, so I would not be offended if the consensus was that this
discussion is not topical enough for the list.  In that case, I would be
happy to move it elsewhere, maybe to its own mailing list.  Just keep in
mind that these features are potentially the most confusing and powerful
language extensions that Python has ever had, and that people (like myself)
are currently writing code that is beginning to make full use of them. 
There are a lot of dusty corners to explore, and it is important for the
principal Python developers to be aware of and to deal with any gremlins
hiding in them.

It would also be nice if some of the examples I'm working on could
eventually become part of the official Python test suite.  If for no other
reason, because the current set of tests is somewhat sparse.

Regards,
-Kevin

--
Kevin Jacobs
The OPAL Group - Enterprise Systems Architect
Voice: (216) 986-0710 x 19         E-mail: jacobs@theopalgroup.com
Fax:   (216) 986-0714              WWW:    http://www.theopalgroup.com

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